1. Technical Field
The present disclosure relates to a gallium nitride based compound semiconductor light-emitting element and also relates to a light source including the light-emitting element (which is typically a white light source).
2. Description of the Related Art
A nitride semiconductor including nitrogen (N) as a Group V element is a prime candidate for a material to make a short-wave light-emitting element, because its bandgap is sufficiently wide. Among other things, gallium nitride-based compound semiconductors (which will be referred to herein as “GaN-based semiconductors”) have been researched and developed particularly extensively. As a result, blue-ray-emitting light-emitting diodes (LEDs), green-ray-emitting LEDs and semiconductor laser diodes made of GaN-based semiconductors have already been used in actual products.
A GaN-based semiconductor has a wurtzite crystal structure. FIG. 1 schematically illustrates a unit cell of GaN. In an AlaGabIncN (where 0≦a, b, c≦1 and a+b+c=1) semiconductor crystal, some of the Ga atoms shown in FIG. 1 may be replaced with Al and/or In atoms.
FIG. 2 shows four primitive vectors a1, a2, a3 and c, which are generally used to represent planes of a wurtzite crystal structure with four indices (i.e., hexagonal indices). The primitive vector c runs in the [0001] direction, which is called a “c axis”. A plane that intersects with the c axis at right angles is called either a “c plane” or a “(0001) plane”. It should be noted that the “c axis” and the “c plane” are sometimes referred to as “C axis” and “C plane”.
As shown in FIG. 3, the wurtzite crystal structure has other representative crystallographic plane orientations, not just the c plane. Portions (a), (b), (c) and (d) of FIG. 3 illustrate a (0001) plane, a (10-10) plane, a (11-20) plane, and a (10-12) plane, respectively. In this case, “−” attached on the left-hand side of a Miller-Bravais index in the parentheses means a “bar” (a negative direction index). The (0001), (10-10), (11-20) and (10-12) planes are c, m, a and r planes, respectively. The m and a planes are “non-polar planes” that are parallel to the c axis but the r plane is a “semi-polar plane”. It should be noted that the m plane is a generic term that collectively refers to a family of (10-10), (−1010), (1-100), (−1100), (01-10) and (0-110) planes.
Light-emitting elements that use gallium nitride based compound semiconductors have long been made by “c-plane growth” process. In this description, the “X-plane growth” means epitaxial growth that is produced perpendicularly to the X plane (where X=c, m, a or r, for example) of a hexagonal wurtzite structure. As for the X-plane growth, the X plane will be sometimes referred to herein as a “growing plane”. Furthermore, a layer of semiconductor crystals that have been formed as a result of the X-plane growth will be sometimes referred to herein as an “X-plane semiconductor layer”.
If a light-emitting element is fabricated as a semiconductor multilayer structure by c-plane growth process, then intense internal electric polarization will be produced perpendicularly to the c plane (i.e., in the c axis direction), because the c plane is a polar plane. Specifically, that electric polarization is produced, because on the c-plane, Ga and N atoms are located at different positions with respect to the c axis. Once such electric polarization is produced in a light-emitting layer (i.e., in an active layer), the quantum confinement Stark effect of carriers will be generated. As a result, the probability of radiative recombination of carriers in the light-emitting layer decreases, thus decreasing the luminous efficiency as well.
To overcome such a problem, a lot of people have recently been making every effort to grow gallium nitride based compound semiconductors on a non-polar plane such as an m or a plane or on a semi-polar plane such as an r plane. If a non-polar plane can be selected as a growing plane, then no electric polarization will be produced in the thickness direction of the light-emitting layer (i.e., in the crystal growing direction). As a result, no quantum confinement Stark effect will be generated, either. Thus, a light-emitting element with potentially high efficiency can be fabricated. The same can be said even if a semi-polar plane is selected as a growing plane. That is to say, the influence of the quantum confinement Stark effect can be reduced significantly in that case, too.
FIG. 4(a) schematically illustrates the crystal structure of a nitride-based semiconductor, of which the principal surface is an m plane, as viewed on a cross section thereof that intersects with the principal surface of the substrate at right angles. The Ga atoms and nitrogen atoms are on the same atomic plane that is parallel to the m plane. For that reason, no electric polarization will be produced perpendicularly to the m plane. It should be noted that In and Al atoms that have been added are located at Ga sites to replace Ga atoms. Even when at least some of the Ga atoms are replaced with In and Al atoms, no electric polarization will be produced perpendicularly to the m plane, either.
The crystal structure of a nitride-based semiconductor, of which the principal surface is a c plane, as viewed on a cross section thereof that intersects with the principal surface of the substrate at right angles is illustrated schematically in FIG. 4(b) just for your reference. In this case, Ga atoms and nitrogen atoms are not present on the same atomic plane that is parallel to the c plane. For this reason, the electric polarization will be produced perpendicularly to the c plane. A c-plane GaN-based substrate is generally used as a substrate to grow GaN based semiconductor crystals thereon. As the positions of the Ga (or In) atomic layer and nitrogen atomic layer, which are parallel to the c plane, slightly shift from each other in the c-axis direction, electric polarization is produced in the c-axis direction.
According to the manufacturing process disclosed in Japanese Laid-Open Patent Publication No. 2009-253164, in the process step of forming a quantum well structure (as an active layer 50) by alternately growing barrier layers 43, 45, 47 and 49 and well layers 44, 46 and 48 on the principal surface of a GaN substrate 40 in order to minimize a variation in the In composition in the thickness direction of the well layers, each of those well layers is formed by growing InGaN, the growing temperature of each of those barrier layers is set to be a first temperature, the growing temperature of each of those well layers is set to be a second temperature that is lower than the first temperature, and the source gas of In starts to be supplied before the source gas of Ga (trimethylgallium) starts to be supplied in growing each well layer.
According to a method for fabricating a nitride semiconductor laser diode as disclosed in Japanese Laid-Open Patent Publication No. 2009-267124, in order to make a nitride semiconductor laser diode in which indium in InGaN well layers can have a more uniform composition, an InGaN thin film, of which the thickness DW1 is smaller than the thickness DW0 of the well layers (i.e., DW1<DW0), is deposited at a temperature T1 by supplying TMG, TMIn and NH3 to the growing furnace in Step S110. This thin film has a thickness of 1 nm. Next, in Step S111, with the TMIn and NH3 still supplied to the growing furnace, the temperature is changed from T1 into T2 (where T1<T2). Then, in Step S112, the temperature is maintained at T2 with TMIn and NH3 supplied to the growing furnace. And in Step S113, the temperature is changed from T2 into T1 with the TMIn and NH3 supplied to the growing furnace.
In the active layer 5 of the light-emitting element disclosed in PCT International Application Publication No. 2007/026767, a delta layer 4 is embedded in a single quantum well layer of InGaN, thus splitting the quantum well layer into two quantum well layers 3A and 3B. To affect the movement of carriers, the delta layer 4 may have a broader band gap than the quantum well layers 3A and 3B. However, unlike the barriers adopted in the known multiple quantum well (MQW) structure, its thickness is set to be approximately 1 nm in order to substantially induce the movement of electrons and holes.
The nitride semiconductor light-emitting element disclosed in Japanese Laid-Open Patent Publication No. 2007-150066 has a nitride semiconductor multilayer portion 6, including at least an active layer 4 with a light-emitting portion, on a substrate 1 and the active layer 4 has a multiple quantum well structure in which InXGa1-xN (where 0<x≦1) well layers 7 and AlyInzGa1-y-zN (where 0≦y<1, 0≦z<1, 0≦y+z<1 and z<x) barrier layers 8 are stacked alternately. And each of those well layers 7 is split by an AlvInwGa1-v-wN (where 0≦v<1, 0 ≦w<1, 0≦v+w<1 and w<x) thin-film barrier layer 7c into at least first and second well layers 7a and 7b. And the thin-film barrier layer 7c is formed to be at least as thick as one atomic layer and have a thickness of 20 Å or less.
Japanese Laid-Open Patent Publication No. 2010-232290 discloses a nitride semiconductor light-emitting diode which includes at least an n-type nitride semiconductor layer, an active layer, and a p-type nitride semiconductor layer and of which the active layer has multiple light-emitting layers including at least two layers with mutually different In mixed crystal ratios that are in contact with each other.
The luminous efficiency achieved by these technologies of the related art still needs to be increased.